On 8th Aug 2019, I bought a China Unicom prepaid data SIM card at Lo Wu Shenzhen Border. It cost me RMB200 for the SIM card while getting a usage credit of RMB100. The phone number was +86. I used a non-PRC passport to register. This package automatically deducts RMB6 for 800MB, which was the data usage incurred on that day. If there's no data usage incurred, it will not deduct any amount. You can even choose a different data package, if you want to. For example, 7 days 14GB for RMB20 or 10GB for RMB for 30 days. Apart from that, every month it will automatically deduct RMB6 as the monthly subscription, despite this being a prepaid SIM card. The shop keeper told us that, if there was no money available in the main account, the card will be automatically expired and unusable. You can find this China Unicom shop right after you go through Lu Wu immigration. As it is just outside of immigration, you can even purchase it even though you are not traveling from Hong Kong to Lo Wu. All you need to do is just walk to the exit of immigration.
I heard that you can purchase a SIM card at the Baiyun Airport Guangzhou. So, when I fly out from Baiyun Airport Guangzhou, I went to the arrival hall to check it out. Indeed, you can purchase a SIM card after you get your luggage and outside the place where you get your bag. The price is the same RMB180 for the SIM card with RMB100 credit. It comes with 3GB data. Will automatically deduct RMB19 per month as your subscription.
When I need to use Facebook & other blocked apps, I just use a VPN. However, a lot of apps in China are tagged to China Mobile.
Although this SIM card is expensive, it's very useful. I would not be able to purchase one if I were in a smaller location in China. From past experience, I was only able to get the SIM card at the airport or border of immigration. The best price was still from Lo Wu, where the monthly deduction was only RMB6. To keep the SIM card active, all you need to do is top up the account to enough to cover the monthly deductions and let it automatically deduct until your next visit to China.
Note: I've previously posted this information on a couple of other sites.